
<PAGE>

                                 UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                 SCHEDULE 13G

                   Under the Securities Exchange Act of 1934

                                (Amendment 1)*

                                 CLARCOR INC.
                               (Name of Issuer)

                                 Common Stock
--------------------------------------------------------------------------------
                        (Title of Class of Securities)

                                   179895107
                            ----------------------
                                (CUSIP Number)


                               December 31, 2000
             -----------------------------------------------------
            (Date of Event Which Requires Filing of this Statement)


Check the appropriate box to designate the rule pursuant to which this Schedule
                                   is filed:

                               [X]  Rule 13d-1(b)
                               [_]  Rule 13d-1(c)
                               [_]  Rule 13d-1(d)


*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).

                               Page 1 of 9 pages
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----------------------                                     -------------------
  CUSIP No. 179895107                   13G                 Page 2 of 9 Pages
----------------------                                     -------------------

------------------------------------------------------------------------------
      NAME OF REPORTING PERSON
 1.   S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

      Liberty Wanger Asset Management, L.P.
      36-3820584
------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
 2.                                                             (a) [_]
                                                                (b) [_]
      Not Applicable
------------------------------------------------------------------------------
      SEC USE ONLY
 3.


------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4.

      Delaware
------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5.
     NUMBER OF
                          None
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6.

     OWNED BY             1,470,000
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7.
    REPORTING
                          None
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8.
                          1,470,000
------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9.
      1,470,000
------------------------------------------------------------------------------
      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
10.
                                                                    [_]
      Not Applicable
------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
11.

      6.0%
------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON*
12.
      IA
------------------------------------------------------------------------------
                     *SEE INSTRUCTION BEFORE FILLING OUT!
<PAGE>

----------------------                                     -------------------
  CUSIP No. 179895107                   13G                 Page 3 of 9 Pages
----------------------                                     -------------------

------------------------------------------------------------------------------
      NAME OF REPORTING PERSON
 1.   S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

      WAM Acquisition GP, Inc.
------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
 2.                                                             (a) [_]
                                                                (b) [_]
      Not Applicable
------------------------------------------------------------------------------
      SEC USE ONLY
 3.


------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4.

      Delaware
------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5.
     NUMBER OF
                          None
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6.

     OWNED BY             1,470,000
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7.
    REPORTING
                          None
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8.
                          1,470,000
------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9.
      1,470,000
------------------------------------------------------------------------------
      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
10.
                                                                    [_]
      Not Applicable
------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
11.

      6.0%
------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON*
12.
      CO
------------------------------------------------------------------------------
                     *SEE INSTRUCTION BEFORE FILLING OUT!
<PAGE>

Item 1(a)      Name of Issuer:

                    Clarcor Inc.

Item 1(b)      Address of Issuer's Principal Executive Offices:

                    2323 Sixth Street, P.O. Box 7007
                    Rockford, Illinois  61125

Item 2(a)      Name of Person Filing:

                    Liberty Wanger Asset Management, L.P. ("WAM")
                    WAM Acquisition GP, Inc., the general partner of WAM
                      ("WAM GP")

Item 2(b)      Address of Principal Business Office:

                    WAM and WAM GP are located at:

                    227 West Monroe Street, Suite 3000
                    Chicago, Illinois  60606

Item 2(c)      Citizenship:

                    WAM is a Delaware limited partnership; and WAM GP is a
                    Delaware corporation.

Item 2(d)      Title of Class of Securities:

                    Common Stock

Item 2(e)      CUSIP Number:

                    179895107

Item 3         Type of Person:

                     (e) WAM is an Investment Adviser registered under section
                         203 of the Investment Advisers Act of 1940; WAM GP is
                         the General Partner of the Investment Adviser.

                               Page 4 of 9 pages
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Item 4         Ownership (at December 31, 2000):

                    (a)  Amount owned "beneficially" within the meaning of rule
                         13d-3:

                         1,470,000

                    (b)  Percent of class:

                         6.0% (based on 24,323,422 shares outstanding as of
                         August 26, 2000)

                    (c)  Number of shares as to which such person has:

                              (i)    sole power to vote or to direct the vote:
                                     none
                              (ii)   shared power to vote or to direct the vote:
                                     1,470,000
                              (iii)  sole power to dispose or to direct the
                                     disposition of:  none
                              (iv)   shared power to dispose or to direct
                                     disposition of:  1,470,000

Item 5         Ownership of Five Percent or Less of a Class:

                    Not Applicable

Item 6         Ownership of More than Five Percent on Behalf of Another Person:

                    The shares reported herein have been acquired on behalf of
                    discretionary clients of WAM.  Persons other than WAM and
                    WAM GP are entitled to receive all dividends from, and
                    proceeds from the sale of, those shares.

Item 7         Identification and Classification of the Subsidiary Which
               Acquired the Security Being Reported on by the Parent Holding
               Company:

                    Not Applicable

Item 8         Identification and Classification of Members of the Group:

                    Not Applicable

Item 9         Notice of Dissolution of Group:

                    Not Applicable

                               Page 5 of 9 Pages
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Item 10        Certification:

                    By signing below I certify that, to the best of my knowledge
               and belief, the securities referred to above were acquired and
               are held in the ordinary course of business and were not acquired
               and are not held for the purpose of or with the effect of
               changing or influencing the control of the issuer of the
               securities and were not acquired and are not held in connection
               with or as a participant in any transaction having that purpose
               or effect.

                               Page 6 of 9 Pages
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                                   Signature
                                   ---------

     After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete and
correct.

Date:  February 14, 2001

               The undersigned corporation, on the date above written, agrees
               and consents to the joint filing on its behalf of this Schedule
               13G in connection with its beneficial ownership of the security
               reported herein.


                               WAM Acquisition GP, Inc.
                                 for itself and as general partner of
                                 LIBERTY WANGER ASSET MANAGEMENT, L.P.


                               By:  /s/ Bruce H. Lauer
                                    -------------------------------------------
                                        Bruce H. Lauer
                                        Senior Vice President and Secretary

                               Page 7 of 9 Pages
<PAGE>

                                 Exhibit Index
                                 -------------

Exhibit 1           Joint Filing Agreement dated as of February 14, 2001 by and
                    among Liberty Wanger Asset Management, L.P. and WAM
                    Acquisition GP, Inc.

                               Page 8 of 9 Pages
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                                                                       EXHIBIT 1

                             JOINT FILING AGREEMENT

     The undersigned hereby agree to the joint filing of the Schedule 13G to
which this Agreement is attached.

               Dated:  February 14, 2001

                               WAM Acquisition GP, Inc.
                                 for itself and as general partner of
                                 LIBERTY WANGER ASSET MANAGEMENT, L.P.


                               By:  /s/ Bruce H. Lauer
                                    -------------------------------------------
                                        Bruce H. Lauer
                                        Senior Vice President and Secretary

                               Page 9 of 9 pages
